Because it's 2012 and everyone with a D next to their name, if elected, will vote the same way every other D votes. There's virtually no such thing as a 'Blue Dog' Democrat anymore.
This is a preposterous and easily disproved statement simply by looking at anything the left-wing of the Democratic Party has pressed in the last 30 years and borders on projection, as it actually
does apply in the reverse to the Republicans. But whatever.
I get it, I really do. You're trying to learn from your left-wing counterparts along with the new class of upstart forum right-wingers like Vosem and Koenkai in the "speaking authoritatively as if your opinions are self evidently true and thus require no explanation" school of posting. But it helps to actually be correct when you do so.